1. Field of the Invention
The present invention relates to a method and related apparatus for improving continuous packet connectivity (CPC) for a user equipment in a wireless communications system, and more particularly, to a method and related apparatus for improving HS-SCCH less operation in CPC for a user equipment in a wireless communications system.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. Through the 3G mobile telecommunications system, a user can utilize a wireless communications device, such as a mobile phone, to realize real-time video communications, conference calls, real-time games, online music broadcasts, and email sending/receiving. However, these functions rely on fast, instantaneous transmission. Thus, targeting at the third generation mobile telecommunication technology, the prior art provides High Speed Package Access (HSPA) technology, which includes High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA), to increase bandwidth utility rate and package data processing efficiency to improve uplink/downlink transmission rate.
For HSDPA and HSUPA, the 3rd Generation Partnership Project (3GPP) provides a continuous packet connectivity (CPC) protocol specification, which includes features that, for user equipments (UEs) in CELL_DCH state, aim to significantly increase the number of packet data users for a cell, reduce the uplink noise rise and improve the achievable download capacity for VoIP.
For an HSDPA UE, the UE can work in a special mode, HS-SCCH less operation, included in CPC. Under this mode, a high speed shared control channel (HS-SCCH) is not transmitted with the first hybrid automatic repeat request (HARQ) transmission. In the HS-SCCH less operation, the UE receives a high speed physical downlink shared channel (HS-PDSCH) according to required control signals received when the HS-SCCH less operation is enabled, such as “channelization-code-set”, “modulation scheme”, “transport-block size”, “UE identity” and etc., for reducing HS-SCCH overhead for a HARQ process, thereby reducing UE power consumption. Note that in the HS-SCCH less operation, the same data is limited to 3 transmissions at most, in other words, the HARQ is limited to 2 retransmissions.
For a detailed HARQ procedure for the HS-SCCH less operation, please refer to associated communications protocol specification of 3GPP. When the HS-SCCH less operation is enabled, the lower layers will provide a MAC-hs PDU at each transmission time interval (TTI), and the UE places the data of the first transmission in a soft buffer, replacing any data previously stored in the soft buffer. Next, the UE decodes the data in the soft buffer for the MAC-hs PDU according to the control signals received when the HS-SCCH less operation is enabled. If the data has been decoded successfully, the UE will report a positive acknowledgement (ACK) to the base station, also known as Node-B, and wait for a new transmission. For the first transmission, if the data has not been decoded successfully, the UE will not report a negative acknowledgement (NAK) to the Node-B and just wait for the second transmission. During the second and the third transmission, the control signals, such as “second or third transmission” and “pointer to the previous transmission”, are transmitted by the HS-SCCH.
Next, for the second transmission, the UE gets the data of the first transmission in the soft buffer according to the control signals, “second or third transmission” and “pointer to the previous transmission”, and combines the data of the first and the second transmission to be a first combined data. Then, the UE decodes the first combined data according to the controls signals in the second transmission. If the first combined data has been decoded successfully, the UE will report an ACK to the Node-B and wait for a new transmission. Otherwise, the UE will store the first combined data in a soft buffer, report a NAK to the Node-B and wait for the third transmission.
Similarly, for the third transmission, the UE gets the first combined data in the soft buffer according to the control signal, “second or third transmission” and “pointer to the previous transmission”, and combines the first combined data with the data of the third transmission to be a second combined data. Then, the UE decodes the second combined data according to the controls signals in the third transmission. If the second combined data has been decoded successfully, the UE will report an ACK to the Node-B. Otherwise, the UE will report a NAK to the Node-B, and wait for a new transmission.
In the prior art, for the third transmission, the UE gets the first combined data in the soft buffer according to the control signal, “pointer to the previous transmission”, combines the first combined data with the data of the third transmission to be a second combined data, and decodes the second combined data for decoding data more efficiently. However, in a poor communication quality environment, the UE may not receive the data of the second transmission, thus the data in the soft buffer obtained according to the control signals will not be the correct first combined data. In this situation, the data of the third transmission will be decoded unsuccessfully, for being combined with the failed decoded data.
In conclusion, in the HS-SCCH less operation, the lost data of the second transmission may result in unsuccessful decoding of the third transmission. In fact, the above condition may happen with the probability 1% at most in a poor communication quality environment.